Method for organizing a mode-locked pulse train by pump modulation

ABSTRACT

The present invention describes a method to achieve stabilization of the repetition rate in a passive harmonic mode-locked fiber laser employing semiconductor saturable absorbers. The pulse organization is accomplished by electrically modulating the amplifier pump source that in turn optically modulates the saturable loss of semiconductor absorber. Moreover owing on an efficient modulation mechanism of the cavity loss, the method can be used to generate an actively mode-lock pulse train. The invention offers the advantages of an actively modulated mode-locked laser while maintaining the simplicity and the cost effectiveness of a passive mode-locked system. We expect that this approach combined with the use of regenerative modulation technique and polarization-maintaining fiber components will permit the generation of the dropout-free pulse trains at gigahertz repetition rates with good long-term stability and minimal cost.

FIELD OF THE INVENTION

The present invention relates generally to mode-locked fiber lasers, andin particular, to stabilization or locking to an electrical signal of apassive harmonic mode-locked pulse train. The fiber laser comprisesmeans for generating a mode-locked pulse train, means for stabilizingsaid mode-locked pulse train to an electrical signal, a semiconductorabsorber, and a pump for optically modulating a saturable loss of asemiconductor absorber to produce said mode-locked pulse train. Theinvention also relates to a method for stabilizing a mode-locked pulsetrain to an electrical signal, in which a saturable loss of asemiconductor absorber is optically modulated by a pump to produce saidmode-locked pulse train. The invention further relates to a system forstabilizing a mode-locked pulse train to an electrical signal,comprising a pump for optically modulating a saturable loss of asemiconductor absorber to produce said mode-locked pulse train.

BACKGROUND OF THE INVENTION

Compact sources generating ultrashort optical pulses near 1550 nmwavelength range are widely regarded as a key enabling technology indeveloping future optical networks. Several transmission schemes, suchas described by Nakazawa et al. in Electron. Lett., vol. 34, pp.907-909, 1998 and Boivin et al. in Photon. Technol. Lett., vol. 11, pp.1319-1321, 1999, have been proposed to increase the system capacity byefficiently using ultrashort pulse generators. Mode-locked fiber lasers,employing rare-earth doped fiber as the lasing material and exploitingvarious active and passive mode-locking techniques provide an attractivesource of ultra-short pulses with tunable repetition rate and tunablelaser wavelength. See for example U.S. Pat. No. 5,008,887 to Kafka andU.S. Pat. No. 5,050,183 to Duling.

Passive mode-locking techniques based on saturable absorber are the mostpromising as it concerns the pulse-width and the simplicity of the lasercavity, as described by Collings et al. in J. Sel. Top. in QuantumElectron., vol. 3, pp. 1065-1075, 1997 and U.S. Pat. No. 6,097,741 toLin et al. A saturable absorber imposes an intensity dependent nonlineareffect on a light beam incident upon it. An incident radiation of lowintensity is absorbed while a high intensity radiation is permitted topass the absorber with much less attenuation. Thus, when used in a lasercavity the saturable absorber will introduce intensity dependent losses.Because the laser tends to operate with minimum cavity loss perround-trip the longitudinal modes of the lasers are locked together inphase corresponding to high intensity short optical pulses in the timedomain. A mode-locking technique which relies upon the use of thenonlinear reflectivity of a semiconductor saturable absorber mirror(SESAM) is attractive because it eliminates the need for critical cavityalignment, it can be designed to operate in a wide spectral range, hasultrafast nonlinear dynamics and relatively large nonlinear reflectivitychanges. Ultrashort optical pulses have been produced with thistechnique using different semiconductor structures and mirror designs.See for example U.S. Pat. No. 5,627,854 to Knox, U.S. Pat. No. 5,237,577to Keller and Zhang et al., Appl. Phys. B, vol. 70, pp. 59.62, 2000.

In order to achieve gigahertz repetition rate a mode-locked laser has tobe operated at a harmonic of the fundamental frequency. It should benoted, however, that due to the long relaxation time of the amplifier,the laser will only saturate in the average power and not individualpulses, as thought by Harvey et al. in Opt. Lett., vol. 18, pp. 107-109,1993. Therefore the output of such a laser suffers increasedpulse-to-pulse instability and supermode competition, thus leading topulse dropouts and repetition rate instability. Harmonic mode-lockingstabilization, a primary condition for telecommunication applications,has been achieved using both active and passive techniques.

Actively mode-locked lasers produce pulses with excellent stability,however pulses are typically much longer than those obtained by passivemode-locked lasers. To further reduce the pulse width, activelymode-locked fiber lasers synchronized to an external clock have beendemonstrated employing soliton pulse shortening, as demonstrated byKafka et al. in Opt. Lett. vol. 14, pp. 1269, 1989 and Carruthers et al.in Opt. Lett., vol. 21, pp. 1927-1929, 1996, or other passive pulseshaping techniques, see Okhotnikov et al., Photon. Tech. Lett., vol. 14,2002. Further details for producing short duration pulses are disclosedin the technical paper of D. J. Jones et. al, Opt. Lett., vol. 21, pp.1818, 1996. Moreover, it was shown that in a harmonically activemode-locked fiber ring laser (Thoen et al., Opt. Lett., vol. 25,948-950, 2000), the amplitude fluctuations can be significantly reduced,thus pulse dropouts are eliminated, owing on optical limiting action oftwo-photon absorption in semiconductor saturable absorbers. However, anactively mode-locked fiber laser requires advanced modulation devicesand driving electronics, resulting in complicated and ultimatelyexpensive laser cavities.

In contrast, Fermann et al., see U.S. Pat. No. 5,414,725 and Okhotnikovet al., see Appl. Phys. B, vol. 72, pp. 381-384, 2001 demonstrated apassive technique to produce ultrashort pulses with stable repetitionrates comparable to that of typical mode-locked lasers. Thestabilization of the repetition rate was achieved by harmonicpartitioning of the laser cavity by a semiconductor saturable absorberwhich is preferentially bleached when the two pulse streams circulatingwithin the main cavity of length n×L, respective subcavity of length Lcollide upon the saturable absorber. While this system proved veryefficient in stabilizing the repetition rate it has the limitation of afixed repetition rate with selection of the positioning of the saturableabsorber and is only adjustable by physically moving the intra-cavityelements defining the main cavity, respective the ‘harmonic’ sub-cavity.

In another attempt, generation of ultrashort pulses with stable andadjustable repetition rate from passively mode locked fiber lasers wasachieved by using a semiconductor saturable absorber with a life-time ofthe order of 10 ns into a fiber lasers with cavity round-trips of theorder of 100 ns, as further disclosed in U.S. Pat. No. 5,701,319.However the pulse jitter was limited to 300 ps and 50 ps for arepetition rate of 20 MHz, respectively 500 MHz. Yet it is believed thatsuch a laser generates a harmonic pulse train with a reduced long-termstability of the repetition rate.

Grudinin et al. suggested, in Electron. Lett., vol. 29, pp. 1860-1861,1993, that soliton interplay, through long-lived acousto-opticinteractions, can stabilize the repetition rate of a passive harmonicmode-locked pulse train. However, this method requires large intracavitypowers to achieve sufficient nonlinearities for stable operation. Thus,cavity lengths of 15 m and longer are required resulting in lowfundamental repetition rates which in turns leads to environmentalinstability and calls for pump levels of a few hundred milliwatts inorder to achieve repetition rates of hundred MHz. Yet, more recently,Gray et al. postulated that phase effects in semiconductor saturableabsorber could lead to pulse repulsion, which provides in turnself-stabilization of the pulse repetition rate, see Opt. Lett., vol.21, pp. 207-209, 1996. However, pulse self-organization provided bysaturable absorber is sensitive to absorber lifetime and uniform pulsedistribution was not observed for absorbers with a carrier lifetime ofless than 500 ps. Moreover the repulsive forces between pulses,responsible for self-organization, are sensitive to amplitudefluctuations, therefore optical limiting has to take place in order tobuild up a stable pulse train.

Recently, generation of an equally spaced soliton pulse train from ashort-cavity harmonic mode-locked fiber laser employing a passive pulseformation mechanism, i.e. semiconductor saturable absorber, has beenachieved by modulating the cavity loss through optical pumping thesaturable absorber by a control beam, as described by Banadeo et al. inOpt. Lett., vol. 25, pp. 1421-1423, 2000. The modulation beam, with awavelength above semiconductor absorber bandgap, was generated byexternally modulating a semiconductor laser placed outward the fiberlaser cavity. It was shown that the time ordering could be dramaticallyimproved by this method given that the modulation beam has a frequencyof a high harmonic of the fundamental repetition rate. The methodresulted in 35-dB suppression of the undesired harmonic modes of a 1.244GHz-pulse train. Thus, it was demonstrated that optical modulation ofthe saturable absorber has the potential to generate a jitter-free pulsetrain with controlled repetition rates and reduced complexity. Roth etal. further developed the method by using a directly modulatedhigh-speed laser diode and implementing a simpler design for internallypumping the saturable absorber, see Electron. Lett., vol. 38, pp. 16-17,2002.

Although the above-described method provides the means to reduce thecomplexity of the laser cavity, the use of an additional semiconductorlaser as pumping sources for providing optical modulation of thesaturable absorber, sets a limit in the attempt of achieving costeffective laser designs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a stable low costharmonic passive mode-locked fiber laser capable of generatingultrashort optical pulses with controlled repetition rate. The presentinvention seeks a more efficient way of controlling the repetition rateof passively mode-locked fiber laser by modulating the light beam usedboth for pumping the laser amplifier and optically modulating thesaturable absorber loss. Yet it is another aim of the present inventionto provide an efficient mechanism to actively mode-lock a fiber laser byoptically controlling the loss introduced by a semiconductor saturableabsorber.

A method according to the present invention is primarily characterizedby that said pump is modulated by an electrical signal. A systemaccording to the present invention is primarily characterized by thatthe system comprises means for modulating said pump by an electricalsignal. A fiber laser according to the present invention is primarilycharacterized by that the fiber laser comprises means for modulatingsaid pump by an electrical signal.

As already stated herein and elsewhere, see B. C. Collings et al., Opt.Lett., vol. 23, pp. 123-125, 1998, a short cavity mode-locked fiberlaser has the advantage of generating ultrashort pulses at very highfundamental repetition rates. This in turn leads to harmonicmode-locking operation with a low number of pulses circulating in thecavity that have increased stability against noise and environmentalfluctuations. On the other hand a short fiber amplifier will notcompletely absorb the available pump power required to provide a certaingain value. Even for an optimized amplifier there is a significantfraction of the incident pump, for example 10%, that is not absorbed. Asa result the residual pump power is eventually lost through the cavityend, i.e. the one containing the saturable absorber. To increase thepump absorption efficiency the saturable absorber mirror can beoptimized to reflect both the pump beam and the signal light. Yetanother way to increase the pump absorption efficiency is to propagatethe pump light in a direction away from the saturable absorber, thusbeing reflected back by a broadband mirror positioned at the cavity endopposing the saturable absorber. However, the pump energy is notcompletely absorbed even after double passing the amplifier section.According to the present invention, the pump beam, and in consequencethe residual pump energy, incident upon a semiconductor saturableabsorber positioned at the amplifier end to passively mode-lock a fiberlaser, is intensity modulated by a RF signal with a frequencysubstantially equal with the repetition rate of the mode-locked pulsetrain. By modulating the cavity loss introduced by the absorber themode-locked pulse train can be synchronized to an external electricalsignal, therefore achieving repetition rate stabilization, without theneed for external components, i.e. modulators. It should be noted thatthe population relaxation time for an rare-earth doped active materialranges from hundreds of microseconds to few milliseconds, therefore anypump variation on a much smaller time scale, for example, microsecond tonanoseconds, will not induce a gain variation. More precisely, the gainwill rather have a constant value set by the average level of the pumpsignal.

The application of the present invention depends on an efficienthigh-speed modulation response of standard high-power pump lasersmodules. Recently, Mohrdiek et al., demonstrated direct intensitymodulation at 3 Gb/s of a 980 nm high-power pump-diode module, (LEOS'98IEEE, pp. 299-300, 1998). Moreover, the intrinsic modulation bandwidthmeasurements showed excellent modulation capabilities extended beyondthe frequency of 10 GHz. Therefore we believe that this invention can besuccessfully accomplished offering a great improvement over today stateof the art ultrashort pulse fiber lasers, in particular due to reducedcomplexity and minimal cost.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the invention is provided by the descriptionof the specific illustrative embodiments and the corresponding drawingsin which:

FIG. 1 shows a preferred embodiment of the present invention.

FIG. 2 is a description of a short-cavity laser set-up in accordancewith the present invention.

FIG. 3 depicts another embodiment of the invention employingregenerative-feedback for setting the repetition rate.

FIG. 4 illustrates an embodiment of a passive harmonic mode-locked fiberlaser having a saturable absorber optically pumped above band-gap by amodulated pump beam. Another saturable absorber is used for passivelystarting the mode-locking process.

FIG. 5 describes an exemplary embodiment of the present invention.

FIG. 6 shows the saturation curves for the semiconductor saturableabsorber responsible for starting the mode-locking.

FIG. 7 reveals the nonlinear reflectivity change of the optical pumpedabsorber versus 980 nm pump intensity.

FIGS. 8 a and 8 b represent typical autocorrelation trace and opticalspectrum of the exemplary laser output.

FIGS. 9 a, 9 b and 9 c shows the digital scope traces, the measured RFspectra and the fast communication analyzer traces of the laser output,without repetition rate stabilization provided by optically pumping thesaturable absorber.

FIGS. 10 a, 10 b and 10 c present the digital scope traces, the measuredRF spectra and the fast communication analyzer traces of the laseroutput, having the optical modulation enabled.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention is presented in FIG. 1.The laser cavity consists of a gain medium 1, e.g. erbium doped fiber, afiber optical coupler 2, i.e. WDM, for coupling a pump beam to the gainfiber and an optical coupler 3 for extracting the laser pulses out ofthe resonator. The cavity ends of the fiber laser are butt-coupled to asaturable absorber 4 and a broadband dielectric mirror 5. The opticalisolator 6 reduces the optical reflections from the output port. Apigtailed semiconductor laser diode 7 is used for pumping the gainmedium, e.g. Er-doped fiber. Although the main embodiments of thepresent invention are discussed here in respect to the erbium-dopedamplifier, other gain media (e.g. ytterbium, thulium, neodymium,praseodymium) can be used, exhibiting different concentration of thedoping atoms, different geometrical parameters, e.g. double-clad fiber,and alternative pumping schemes.

The saturable absorber 4 consist in general of a semiconductor materialwith a band-gap energy in the vicinity of laser wavelength that isintegrated with a semiconductor mirror designed to provide a certainreflectivity for the laser radiation. The semiconductor absorber can be,for example, InGaAsP material with a band-gap of 1550 nm. Thesemiconductor mirror comprises, for example, a certain number ofInP—InGaAs layers designed to offer a high reflectivity over a largespectral range extending around the laser wavelength. Alternatively thesaturable absorber can be used in transmission, in this case thesemiconductor mirror is not required. Without going into details, it isnoted that various absorber designs, including but not limited tomultiple-quantum-well absorbers, bulk absorbers, organic dyes, excitonicbased absorbers, saturable-Bragg-reflectors, anti-resonant Fabry-Perotsaturable absorbers mirrors, dispersive saturable absorber mirrors,wafer-bonded absorbers, metallic mirror based absorbers, can be usedwithout departing from the scope of the present invention.

According to the present invention the light beam emitted by the pumpdiode 7, with a wavelength above the energy band gap of the absorber, isused both for exciting the rare-earth atoms, thus resulting instimulated emission of light at around 1550 nm, and to optically pumpthe semiconductor absorber 4. The pump diode is directly modulated by anelectrical signal consisting of a continuous component (DC) and amodulation signal (RF), having for example a sinusoidal or rectangularshape, with a frequency equal with the repetition rate of the harmonicmode-locking pulse train. A bias-T 8 combines the DC and RF signals usedfor modulation the pump source. Driving the pump diode only by the DCcomponent causes the fiber laser to operate in a passive harmonicmode-locking regime induced by the saturable absorber. Applying amodulation signal with a frequency equal with one of the cavityharmonics will modulate the loss introduced by the saturable absorberthus locking the positions of the harmonic mode-locked pulses train tothe time window when the modulation is applied. The levels of theapplied DC and RF signals should be optimized according to two designrules. First, the available pump power increases with the DC signal fora given amplitude of the RF signal. On the other hand, if the pump powerincident on the absorber has a high DC component, it will continuouslysaturate the absorber therefore reducing the modulation index of theabsorber, the driving force responsible for self-starting andstabilizing the mode locked pulse train. At limit the pump laser can bedriven only by the RF signal with the DC component in the thresholdvicinity therefore maximizing the modulation index of the absorber inexpense of decreasing the pump energy for a given modulation amplitudeof the drive signal. Since the amplifier dynamics cannot follow the pumpvariation, the gain will be determined by the average optical power ofthe pump beam that in turn depends on the modulation amplitude and theduty cycle. Moreover, the saturation features of the absorber, i.e.optical modulation index, can be optimized by coating the absorbersurface to ensure an efficient mode-locking and stabilization mechanismsfor a given pump power and pump modulation amplitude. Alternatively byoptimizing the saturable absorber and the laser cavity the method can beused for generation a pure actively mode-locked pulse train.

In attempt to further reduce the laser complexity another embodiment ofthe present invention is described in the FIG. 2. Here the Er-dopedfiber end opposing the saturable absorber 4 is coated with a dichroicmirror 9 having a high reflectivity at the signal wavlength and a hightransmissivity at the pump wavelength. The pigtailed pump 7 is coupledto the Er-doped fiber through the coated end via a WDM coupler 2 and arotary splice 10 or a glued splice. The same port is used to couple thesignal light out of the laser cavity. The above described laser systemoffers several important improvements over the previous design such as:increased repetition rate for the same amplifier length by eliminatingthe intra-cavity output coupler, increased laser efficiency by usingonly one output port, and reduced price.

It is well know that temperature and environmental fluctuation leads tochanges of the fundamental repetition rate of mode-locked fiber laser.Therefore, an active technique for organizing the pulse train requiresfine-tuning of modulation frequency in order to eliminate theundesirable phase shift between the modulation and the mode-locked pulsetrain. Employing a regenerative mode-locking technique that extracts aclock signal from the output of the laser and use it to drive themodulator has already been proposed in order to solve this problem, seefor example U.S. Pat. No. 5,598,425 to Jain et al. and Margalit et al.,in Photon. Technol. Lett, vol. 10, pp. 337-339, 1998. An embodiment of aregenerative mode-locking technique implemented in accordance with thepresent invention is described in FIG. 3. The optical coupler 11 splitsthe laser output so that a part of the laser power feeds a clockextraction circuit 12 that, for example, includes a high-speedphoto-detector, a narrow-band electrical filter and an electricalpre-amplifier (not shown). The clock extraction circuit generates asinusoidal clock signal with a frequency corresponding to a highharmonic frequency of the laser cavity. The clock signal under-goesphase adjustment through the phase shifter 13, is amplified by anelectrical amplifier 14 and is used to modulate the pump source 7. Inthis way intensity modulation of the light is carried out in the cavityat a frequency locked to the clock signal. Although the above mentionedlaser design leads to a stable optical pulse train with a highrepetition rate, the repetition rate slightly varies owing tofluctuation in the cavity length. This problem can further be solved byemploying a technique to stabilize the cavity length as described inseveral publications, see for example Nakazawa et al., Photon. Technol.Lett., vol. 12, pp. 1613-1615, 2000.

The selection of a particular saturable absorber in accordance with thepresent invention should address in general two design requirements: thesaturable absorber 4 has to provide an efficient passive mechanism tomode-lock the laser and to have a high modulation index when it isoptically modulated by the residual pump beam. In FIG. 4 an alternativeembodiment of the invention is proposed aiming to easy the designconstrains of the saturable absorber by using two saturable absorbersindependently optimized to provide an efficient mode-locking mechanism,absorber 15, respectively a sufficient modulation index by opticalpumping, absorber 16.

Several experiments were conducted to prove the main idea behind thisinvention by verifying that a saturable absorber of a certain kind,optically pumped by a light beam with properties identical with thelight used to pump the laser amplifier, can stabilize passive harmonicmode-locking. An exemplary embodiment of the present invention is shownin FIG. 5. Here the passive harmonic mode-locked fiber laser consist ofa gain medium 17, i.e. erbium doped fiber, a fiber optical coupler 18,i.e. WDM, for coupling a pump beam to the gain fiber, another fiberoptic coupler 19 for coupling to the cavity a controlled beam with awavelength identical with the pump beam, an optical coupler 20 forextracting the laser pulses out of the resonator, a saturable absorbers21 designed for efficiently starting the mode-locking process and yetanother saturable absorbers 22 that is optically pumped by a controlbeam. The laser is pumped with a single-mode pig-tailed laser diode 23.A similar laser diode 24 is directly modulated by an electrical signalgenerating a control beam used to pump the saturable absorber 22.

In the performed experiment the erbium-doped fiber with a length of 1 mhad an unpumped loss of 38 dB/m at 1535 nm, a core diameter of 6.2 μm, anumerical aperture NA=0.23 and normal group velocity dispersion (GVD) of+0.01 ps²/m at 1560 nm. Referring to FIG. 5, it can be seen that thecavity includes means for focusing the laser beam onto the two saturableabsorbers, i.e. lenses 25-28. It is important that the focal points ofthe lenses 25 and 28 coincide with the positioning of the saturableabsorbers 21, 22 thus the absorber is efficiently saturated. The spotsize on the saturable absorber 21, 22 can be changed, thus adjusting thesaturation fluence of the absorber, by varying the lens position and byusing lenses with different focal lengths. The fiber end wasangle-cleaved to eliminate the influence of the Fresnel reflection.

For the exemplary study the saturable absorbers 21, 22 comprise severalInGaAsP quantum-wells, latticed matched to InP, grown on top of a highreflective semiconductor mirror. The mirror consists of a periodic stackof alternating material layers of GaInAs—InP, i.e. Distributed BraggReflector (DBR). A similar structure was presented by Ning et al. inElectron. Lett., vol. 37, pp. 375-376, 2001.

The two saturable absorbers were independently designed to optimize thepassive mode-locking and to stabilize the repetition rate. The saturableabsorber 21 is responsible for efficiently starting the mode-lockingprocess while introducing minimal nonsaturable loss. FIG. 6 shows thenonlinear reflectivity change of this absorber measured as a function ofthe incident pulse fluence at the wavelength of 1560 nm. This devicecomprises, for example, seven quantum-wells and has a maximumreflectivity change of up to 3% while the non-saturable losses arelimited to less than 10%, including the DBR loss. On the other hand thesaturable absorber 22 should provide a high enough reflectivity change(modulation index) by optical pumping with a 980 nm modulated beam.Therefore this absorber consists of six groups of seven InGaAsPquantum-wells, resembling the same material design as the mode-lockingabsorber. As a result, shown in FIG. 7, an increase of the reflectivitychange by a factor of about 4 was obtained.

Initially, the laser was optimized for passive mode-locking withmodulation signal provided by the pump source 24 off. Optimal alignmentresulted in self-starting mode-locking provided by the saturableabsorber 21. Increasing the pump power the laser operates with multiplepulses in the cavity. The multiple pulsing behaviour depends on thecavity loss and the nonlinear loss experienced by the saturableabsorber. The laser produces soliton-shaped like pulses having apulse-width of about 550 ps independent on pump power. Theautocorrelation trace and the optical spectrum of the mode-locked pulsesare presented in FIG. 8 a, respectively FIG. 8 b. The laser occasionallyexhibited pulse self-organization, however, the pulses were not firmlytied to periodic locations but rather were bunching together, see FIG. 9a. Here for clarity of the presentation the pump power level is set toproduce harmonic mode-locked pulses running in the 6th harmonic (81.4MHz) of the fundamental repetition rate. FIG. 9 b reveals the RFspectrum of the pulse train consisting of six optical pulses circulatingin the cavity. Without optically pumping the saturable absorber 22 thesuppression of the cavity fundamentals was almost inexistent.Furthermore, the pulse spacing can vary by more than 250 ps from pulseto pulse as the measurements performed with a high-speed communicationanalyzer show in FIG. 9 c. Thus it is concluded that the passiveharmonic mode-locking can not generate a laser pulse train with a stablerepetition rate.

Then with the laser diode 24 optimally modulated by a RF signal at thesixth harmonic of the fundamental frequency, a strong pulse ordering canbe observed clearly in the time domain. The positions of the pulses isdetermined by the low-loss time window defined by the optically pumpedsaturable absorber. FIG. 10 a shows that the pulses were organized instable pulse streams with regular intervals between pulses correspondingto harmonic modulation frequency. Indeed the measured RF spectrumpresented in FIG. 10 b demonstrates a suppression of lower cavityharmonics at least about 55 dB, which indicates uniform harmonic modelocking with highly periodic pulse train. Since the suppression of theunwanted harmonics in RF spectrum deteriorates to 37 dB with one pulsedropout per round trip, as thought by Bonadeo in Opt. Lett., vol. 25,pp. 1421-1423, 2000, this laser is free of pulse dropouts and has ahighly stable supermode. During the optimization of the modulationparameters, no changes in pulse shape were observed. Furthermore, theimprovement of the timing jitter with optical modulation on is presentedin FIG. 10 c. The peak-to-peak jitter was <30 ps, which represents anear 10 dB improvement over the purely passive harmonic mode-lockedpulse train. The plot also indicates that there are almost nopulse-to-pulse energy fluctuations. Since no active cavity lengthcontrol circuit was implemented, it is believed that the jitter is mostprobably dominated by environmental fluctuations of the cavity length.The non-polarization-maintaining design of the laser cavity alsocontributes to the degradation of the long-term stability. As themodulation frequency increases, the ability of the modulation tostabilize a harmonic mode-locked pulse train should increase, since timewindow corresponding to the maximum level of the modulation signal getsshorter. Thus, it is believed that higher modulation frequencies willfurther reduce the timing jitter. After the modulation is turned on, ittakes a certain time for the pulses to be synchronized to the externalclock signal. Indeed increasing the modulation factor for the controlbeam results in shorter stabilization time.

The above described laser configuration allows for independentoptimization of the modulation conditions and the amplifier gain inexpense of using two distinct pump sources and saturable absorbers formode-locking and optical modulation.

Yet another variation of the present invention consists of replacing the980 nm pump source, with a source generating light at the wavelengthclose to 1480 nm. In this case the heating effects due to opticalabsorption of 980 nm light by InP material is avoided in the expense ofa relatively higher price of 1480 nm pump sources.

The system of the present invention allows for various modifications ofthe design, i.e. ring laser cavity, “Sigma” laser, all PM-fiber laser,cavity feed-back, as well as other configurations common in the art,without departing from the spirit and scope of the invention. It isintended that the full measure of the invention be determined withreference to the following claims.

1. A method for stabilizing a mode-locked pulse train to an electricalsignal, in which a saturable loss of a semiconductor absorber isoptically modulated by a pump to produce said mode-locked pulse train,wherein said pump is modulated by an electrical signal.
 2. The methodaccording to claim 1, wherein said electrical signal comprises a highfrequency signal.
 3. The method according to claim 2, wherein thefrequency of said modulating high frequency signal is substantially thesame as the repetition rate of said mode-locked pulse train.
 4. Themethod according to claim 1, wherein said electrical signal alsocomprises a DC-component.
 5. The method according to claim 2, whereinthe frequency of said modulating high frequency signal is substantiallythe same as the repetition rate of said mode-locked pulse train.
 6. Themethod according to claim 1, wherein the light beam emitted by the pumpand having a wavelength above the energy band gap of the semiconductorabsorber is used both for exciting the rare-earth atoms and to opticallypump the semiconductor absorber.
 7. The method according to claim 1,wherein two semiconductor absorbers are used independently, wherein thefirst semiconductor absorber is used for starting the mode-lockingprocess, and the second semiconductor absorber is used for achievingsufficient modulation index by optical pumping to synchronize themode-locked pulse train to external RF signal.
 8. The method accordingto claim 1, wherein the loss introduced by said semiconductor absorberis optically controlled and produce active mode-locking synchronized toexternal RF signal.
 9. The method according to claim 1, wherein asinusoidal clock signal with a frequency corresponding to a highharmonic frequency of the laser cavity part is generated from themode-locked pulse train, wherein the clock signal is used to modulatethe pump source.
 10. A system for stabilizing a mode-locked pulse trainto an electrical signal, comprising a pump for optically modulating asaturable loss of a semiconductor absorber to produce said mode-lockedpulse train, wherein the system comprises means for modulating said pumpby an electrical signal.
 11. The system according to claim 10, whereinsaid electrical signal comprises a high frequency signal.
 12. The systemaccording to claim 10, wherein said electrical signal also comprises aDC-component.
 13. The system according to claim 10, wherein it comprisestwo semiconductor absorbers, wherein the first semiconductor absorber isarranged to be used for the mode-locking, and the second semiconductorabsorber is arranged to be used for achieving sufficient modulationindex by optical pumping.
 14. The system according to claim 10, whereinit comprises means for optically controlling the loss introduced by saidsemiconductor absorber to actively mode-locking said pulse train. 15.The system according to claim 10, wherein it comprises means forgenerating a sinusoidal clock signal with a frequency corresponding to ahigh harmonic frequency of the laser cavity part from the mode-lockedpulse train, wherein the clock signal is used to modulate the pumpsource.
 16. A fiber laser comprising means for generating a mode-lockedpulse train, means for stabilizing said mode-locked pulse train to anelectrical signal, a semiconductor absorber, and a pump for opticallymodulating a saturable loss of a semiconductor absorber to produce saidmode-locked pulse train, wherein the fiber laser comprises means formodulating said pump by an electrical signal.